As an emission type electronic displaying device, there is an electroluminescence device (ELD). As elements constituting the ELD, there is an inorganic electroluminescent element or an organic electroluminescent element (hereinafter referred to also as organic EL element). The inorganic electroluminescent element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element.
An organic electroluminescent element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several volts to several decade volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
An organic EL element for practical use is required which efficiently emits light with high luminance at a lower power. For example, in Japanese Patent No. 3093796 is disclosed an element with long lifetime emitting light with high luminance in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound, in Japanese Patent O.P.I. Publication Nos. 63-264692 is disclosed an element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound, and in Japanese Patent O.P.I. Publication Nos. 3-255190 is disclosed an element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye.
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (see M. A. Baldo et al., Nature, 395, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made (see M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000) or U.S. Pat. No. 6,097,147).
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination.
When a phosphorescent compound is used as a dopant compound, it is necessary that a host compound have the emission maximum-providing wavelength in wavelength regions shorter that that of the phosphorescent compound. However, it has been found that there are other requirements to be satisfied.
There are an example (see for example, Applied Physics Letters, 77, 6, p. 904-906 (2000)) in which a phosphorescent compound emitting a green light is used as a dopant compound and an electron transporting material is used as a host compound; an example (see for example, U.S. Ser. No. 2002/0034656 A) in which a phosphorescent element is manufactured employing as a host compound, a carbazole derivative such as CBP; and an example (see for example, Japanese Patent O.P.I. Publication No. 2001-313178, Japanese Patent O.P.I. Publication No. 2001-313179, and Japanese Patent O.P.I. Publication No. 2002-184581) in which a phosphorescent element is manufactured employing as a host compound, an amine type carbazole derivative. However, at present, a more complex structure is necessary to manufacture a stable element sufficient to be practically used (see for example, Pioneer Gijutsu Johoshi, 11, 1, p. 13-20).
There is an example in which a phosphorescent compound emitting a blue to blue-green color light is used as a dopant compound and a carbazole derivative such as CBP is used as a host compound, but the external qauntum efficiency of this example is around 6%, which provides unsatisfactory results (see for example, Sixty second OyobutsuriGakkai GakujutsukoenKai Yokoshu 12-a-M8), and there is room to be improved. With respect to a host compound in the light emission layer or a compound (for example, a hole transporting material, an electron transporting material or a hole blocking material) in a layer adjacent to the light emission layer in a phosphorescence organic EL element emitting a blue light, it is required not only to have the lowest excited triplet level identical to or higher than that of a phosphorescent compound but to have high mobility. However, at present, an effective means has not been found, and a blue light-emitting element with high efficiency has not been found.